The invention relates to a method and a device for detecting the presence or absence of one or more of a fixed number of certain frequencies within a pulse code modulated (PCM) signal, the signal to be investigated being multiplied by each of two reference series for each of the said certain frequencies.
A method and device of this kind are known from Proudfoot U.S. Pat. No. 3882283 issued May 6, 1975.
The method and device are applied in telecommunication exchanges in detecting voice-frequency signalling signals, such as different frequencies corresponding to digits on pushbuttons or on a dial of a telephone set contained in a telecommunication channel itself or in a separate signalling channel.
As detection devices for signalling purposes are widely used in telecommunication exchanges, it is clearly desirable that each detection device is used as efficiently as possible, so that space can be saved in the construction of the exchange and an economically more attractice solution is found. On the other hand, the standards to be met by the detection devices have been laid down in CCITT-recommendations Q440-Q458, which show that detection has to take place within a rather short time even if the frequencies received are somewhat different from the ones expected.
Although the device described in the above-mentioned patent is reasonably insensitive to inaccuracies in the frequency of the signals received, the components of the device are not used very efficiently. An improvement has been realized with the method and device described in applicants' asignees Drukarch U.S. Pat. No. 4068309 issued Jan. 10, 1978, but the latter has the drawback of being sensitive to frequency deviations and level variations of the signal. Both devices have the disadvantage that they cannot properly handle the occurring sum and difference frequencies.
The object of the invention is to provide a method and device which, starting from those described in the Proudfoot U.S. Pat. No. 3882283, yield a more efficient use of the equipment and more reliable detection.